The recent boom in multi-function portable electronic equipments and the increasing need for wireless sensor networks for the development of smart environments raise the problem of compact and/or flexible energy storage. Designing efficient miniaturized energy storage devices that can achieve high energy delivery or can harvest at high discharge rates with a lifetime that matches or exceeds that of the machine being powered remains a challenge. The integration of the storage element as close as possible to the electronic circuit (directly on a chip or within a flexible substrate, for example, including clothing) is another challenge. Since electrochemical energy storage in batteries occurs by volumetric reactions, the charge-discharge rate and specific power of the best (Li-ion) batteries are limited by the rate of solid-state diffusion. The redox reactions and expansion-contraction of the active material limit the lifetime to hundreds or thousands of cycles. These problems can only be partially resolved by using nano-structured materials. The properties of thin-film batteries, despite their excellent energy per unit volume, drop dramatically in the micro-scale range.
Electric double-layer capacitors (EDLCs), also called supercapacitors, store energy using an accumulation of ions of opposite charge in a double layer at electrochemically-stable high specific surface area electrodes. EDLCs do not have a conventional dielectric. Rather than two separate plates separated by an intervening substance, these capacitors use “plates” that are in fact two layers of the same substrate, and their electrical properties, the so-called “electrical double layer”, result in the effective separation of charge despite the vanishingly thin (on the order of nanometers) physical separation of the layers. The lack of need for a bulky layer of dielectric permits the packing of “plates” with much larger surface area into a given size, resulting in extraordinarily high capacitances in practical-sized packages.
In an electrical double layer, each layer by itself is quite conductive, but the physics at the interface where the layers are effectively in contact means that no significant current can flow between the layers. The high surface to volume ratio of the active material promotes the energy and power densities of EDLCs, and is further enhanced in micro-supercapacitors. By offering fast charging and discharging rates, and the ability to sustain millions of cycles, electrochemical capacitors bridge the gap between batteries, which offer high energy densities but are slow, and conventional electrolytic capacitors, which are fast but have low energy densities.
EDLCs have much higher power density than batteries. In fact, existing EDLCs have energy densities that are perhaps 1/10th that of a conventional battery, their power density is generally 10 to 100 times as great (see, e.g., FIG. 1).
Typical materials used for supercapacitors have been limited to activated carbon, graphene, carbon nanotubes, carbon aerogels, and certain conductive polymers. However, few studies have been performed on nanoparticles that do not have narrow pores, unlike activated carbons or nanotubes, wherein the transport of ions may be the rate controlling factor limiting the charge/discharge rate.
Micro-supercapacitors have yet to be described which exhibit the combination of high capacitance, especially high power, and high discharge rates necessary for wide-scale commercial use. There is a clear need in the market for devices that are capable of exhibiting high capacitance levels wherein the powers per volume that are at least comparable to electrolytic capacitors, the energies per volume are higher than electrolytic capacitors, and which exhibit discharge rates significantly higher than conventional supercapacitors. Such devices are available through the present invention.
While exploring the properties of materials of a genus of materials identified herein and hereinafter described as alliform carbon, the present inventors have also discovered properties that, in addition to solving the problems just described, open a wide range of new application possibilities. These new applications include the use of alliform carbon particles in activated carbon electrodes, batteries, and current collectors (and/or the interface of EDLCs therewith), where the particle properties provide for enhanced performance of those devices. Alliform carbon particles also can be used in binderless EDCLs and in conductors and integrated energy storage devices on flexible substrates such as paper, plastics, and woven and non-woven fabrics. This latter discovery—the ability to design functional electrical storage devices in woven and non-woven fabrics—opens the possibility of functional fabrics and medical treatments that provide significant advantages over existing technologies. Each of these options is described below.